Population dynamics of Clausocalanus furcatus (Copepoda, Calanoida) in the northern Gulf of Mexico

Copepods are important components of marine ecosystems. Understanding copepod population dynamics can help interpret variations in both primary producers and higher trophic levels. Egg production, stage duration, and stage-specific mortality rates are key parameters describing copepod population dynamics. Estimation of stage-specific mortality is complicated due to a complex life history, patchiness, and sampling biases. This study was undertaken to quantify the population dynamics of the copepod Clausocalanus furcatus in the northern Gulf of Mexico and to assess the utility of available mortality estimation methods in a highly advective environment. Zooplankton samples were taken every 12h from March 18--April 6, and May 15--June 9, 2003 from an offshore petroleum platform using a 153mum net and a 30L Niskin bottle to characterize the mesozooplankton assemblage. Incubation experiments were conducted during June--July 2002, March--April 2003, and May--June 2003 to measure egg production rates and stage durations. Stage-specific mortality rates were estimated using the horizontal life table (HTL), vertical life table (VTL), quadratic programming method (QPM) and inverse matrix method (IMM). Mesozooplankton communities in the study area were influenced by the Mississippi River plume. Field estimates of the mean egg production rate of C. furcatus were lower than measurements from lab incubation experiments. Egg production rates did not appear to be limited by food availability. A complete generation time ranged from ∼13--20d. Early naupliar stages had shorter durations than late copepodite stages. Comparisons among HTL, VTL, QPM and IMM showed that the HTL and VTL had the disadvantage of producing negative mortality estimates, while the QPM likely overestimated mortality rates. Simulation experiments indicated that variability in stage abundances was a key factor affecting estimates of copepod mortality by the QPM and IMM techniques. Neither the QPM nor the IMM performed well when stage abundance variability was high. IMM estimates of instantaneous egg mortality rates were 1.30d-1 in March--April and 1.60d-1 in May--June. While instantaneous mortality rates for NI-CIV stages ranged from 0.02 to 0.18d-1. Simulated populations using the mortality rates estimated from the IMM technique were consistent with observed field population trajectories.